A glycated protein is a substance which is produced by the non-enzymatic and irreversible binding of the amino group of an amino acid constituting a protein, with the aldehyde group of a reducing sugar such as aldose. See e.g., U.S. Pat. No. 6,127,138. Such a non-enzymatic and irreversible binding reaction is also called “Amadori rearrangement,” and therefore the above-mentioned glycated protein may also be called “Amadori compound” in some cases.
Nonenzymatic glycation of proteins has been implicated in the development of certain diseases, e.g., diabetic complications and the aging process (Takahashi et al., J. Biol. Chem., 272(19):12505–7 (1997); and Baynes and Monnier, Prog. Clin. Biol. Res., 304:1–410 (1989)). This reaction leads to dysfunction of target molecules through formation of sugar adducts and cross-links. Considerable interest has focused on the Amadori product that is the most important “early” modification during nonenzymatic glycation in vitro and in vivo.
Various assays for glycated proteins are known. For example, U.S. Pat. No. 6,127,138 discloses that a sample containing a glycated protein is treated with Protease XIV or a protease from Aspergillus genus, thereafter (or while treating the sample with the above protease) FAOD (fructosyl amino acid oxidase) is caused to react with the sample so as to measure the amount of oxygen consumed by the FAOD reaction or the amount of the resultant reaction product, thereby to measure the glycated protein.
In another example, U.S. Pat. No. 6,008,006 discloses that the amount of glycated proteins in a sample can be quantified by reacting the sample with first a reagent which is a combination of a protease and a peroxidase and second with a ketoamine oxidase. U.S. Pat. No. 6,008,006 also discloses a kit which contains the combined peroxidase/protease enzyme reagent and also the ketoamine oxidase.